When you have been arrested for the suspicion of having committed a criminal offense, you will be given a court date on which you must appear before a Judge. This first court appearance is called the Arraignment. The arraignment is when the court will read you your rights, and the charges against you. The prosecutor will also give you an offer. You will then be asked to enter a plea of guilty or not guilty.
Many people are overwhelmed by the whole process and want it to be over as soon as possible. Therefore, they take the offer the prosecutor gives them that first day in court and finish their case. Although, it is a nerve wrecking experience, it is a bad idea to take the first plea that is offered without knowing your rights. There may be defenses available to you, and some weaknesses in your case that could leave a lesser charge or even a dismissal.
Without an attorney, you would not be aware of what the right offer is and what you should accept before entering a guilty plea. You may even be entering a premature plea when you could have gotten a better deal with some negotiation and bargaining.